The present invention relates to a system for controlling both an engine having high torque fluctuations, when the ON/OFF state of a control valve disposed in an intake port is switched, and an automatic transmission connected to the engine.
As is well known in the art, an automatic transmission for vehicles is set to a predetermined one of gear stages when the power transmission path of a gear train is changed by a friction device including clutches and brakes. This friction device has its load (or torque) capacity varied with not only the coefficients of friction and the diameters of friction members but also the pressure of engagement (i.e., the line pressure). This oil pressure is controlled in accordance with the load to be applied to the friction device. When in shifting, on the other hand, not only the engine speed but also the speeds of various rotating members such as gears also vary. For this shift, therefore, the friction device is caused to slip by changing the oil pressure applied to them gently for a predetermined time period so that the inertial energy accompanying the change in the rotational speeds may be absorbed to prevent an abrupt change in the torque of an output shaft, namely the shift shocks.
The aforementioned engagement pressure is controlled through a change in the level of pressure regulation by applying a pilot pressure corresponding to the engine load to a regulator valve for regulating the oil pressure generated by a hydraulic pump. Specifically, either the throttle pressure to be fed from a throttle valve having its regulation level varied by a throttle cam or the throttle pressure sent from a linear solenoid valve to be controlled according to the throttle opening is applied as the pilot pressure to the regulator valve to change the regulation level. On the other hand, the oil pressure for engaging the friction device may be controlled when in shifting by applying either the oil pressure (i.e., the line pressure) regulated by the aforementioned regulator valve or the oil pressure coming from an accumulator control valve to the back pressure chamber of an accumulator. Incidentally, an example of changing the regulation level by using the linear solenoid valve is disclosed in Japanese Patent Laid-Open No. 215157/1987.
In the aforementioned ordinary control method existing in the prior art, the line pressure will rise as the throttle opening increases, so that the torque capacity of the friction device is augmented whereas the oil pressure in shifting remains at a relatively high level. If, on the contrary, the throttle opening decreases so that the engine torque falls, the line pressure also drops, and then the engagement or release of the friction device in shifting is timed, as expected, not to deteriorate the shift shocks.
Incidentally, the engine connected to the automatic transmission does not always have its output torque changed continuously. In case, therefore, the engine torque change is discontinuous, special controls are required.
The engine falling in this category can be exemplified by the (lean combustion) engine which adopts a lean combustion system, as disclosed in Vol. 38, No. 9 of "Automobile Technology". In this lean combustion engine, an improvement in the fuel economy under a light load and a high output under a high load are made compatible by keeping an air/fuel ratio at a high level while the throttle opening is at or below a predetermined level (i.e., under the light load). In a lean range (having a higher air/fuel ratio), however, it is required to stabilize the combustion in the engine cylinder and to improve the combustion efficiency. For this requirement, the bisected intake port has its one half shaped into a helical swirl port and its other equipped with a control valve (e.g., a swirl control valve, as will be abbreviated into "SCV"). Under a light load, this swirl control valve is closed to establish a swirling flow in the cylinder to ensure the lean combustion. Since, however, the suction is restricted with the swirl control valve being closed, a sufficient output performance is achieved at a throttle opening larger than a predetermined value (i.e., under a high load) by opening the swirl control valve and by reducing the air/fuel ratio to the stoichiometric level or the power level.
FIG. 14 is a diagram plotting the relations between the output torque of the aforementioned lean combustion engine and the throttle opening. A thick solid line appearing in FIG. 14 indicates the actual engine torque. Moreover, a line 1 indicates the torque characteristics when the air/fuel ratio is as high as 21 for the lean combustion, in which the aforementioned swirl control valve is closed. On the other hand, a line 2 or 3 indicates the torque characteristics for a relatively lean combustion having an air/fuel ratio of 17 or 16. Moreover, a line 4 indicates the torque characteristics for the stoichiometric combustion having an air/fuel ratio of 14.5. Still moreover, a line 5 indicates the torque characteristics for the power combustion having an air/fuel ratio of about 12.5.
As shown in FIG. 14, the aforementioned lean combustion engine is subjected to a lean combustion, in which the air/fuel ratio is set at about 21 with the throttle opening T.sub.A being no higher than T.sub.A1, as indicated in FIG. 14. The engine torque is continuously increased by changing the air/fuel ration gradually to 7 or 16 while the throttle opening T.sub.A is at a level between T.sub.A1 and T.sub.A2 of FIG. 14. When the throttle opening T.sub.A exceeds T.sub.A2, the engine torque in the wide opening range is retained partly by opening the swirl control valve and partly by reducing the air/fuel ratio to the power level. In the burning state having the swirl control valve opened, however, the engine torque is discontinuously changed together with the air/fuel ratio between T.sub.1 and T.sub.2, as seen from FIG. 14.
In the prior art, however, the switching of the ON/OFF of the swirl control valve and the shift of the automatic transmission are accomplished independently of each other. In case these operations take place simultaneously, the shift shocks are intensified due to the overlap of the changes in both the engine torque caused by switching the swirl control valve and the output shaft torque caused by the inertial torque in shifting. In case, moreover, the swirl control valve is switched, the engine torque is highly changed even if the throttle opening does not change, as has been described hereinbefore. In the conventional method, by which the line pressure and the accumulator back pressure are controlled according to the throttle opening, these two pressures will become unsuitable for the engine torque (or the input torque to the automatic transmission). As a result, if a shift occurs in this state, its shocks may grow excessive, and still the worse the friction members may lose their durability.
In an engine enabled to change its output characteristics by changing an air/fuel ratio, on the other hand, the air/fuel ratio is reduced to stabilize the combustion if the engine water temperature is low. If the depression rate of an accelerator pedal is high, the air/fuel ratio may be reduced so as to provide high output characteristics. In this engine, at least two output characteristics can be set for one throttle opening. If, therefore, the line pressure of the automatic transmission or the engagement pressure of the friction device is regulated on the basis of the throttle opening, the oil pressure may relatively drop to intensify the shift shocks.